The invention relates to an extensometer for measuring movements in rock structures and in foundations, particularly the movement of layers of rock, consisting of an anchor-type device to be secured at the measuring point, a measuring rod connected therewith, freely movable in an encasing tube and bearing a pick-up element at its free end, and an extensometer head connected with the encasing tube and secured at the pick-up element of the change in the position of the pick-up element of the measuring rod in relation to the extensometer head being measurable (hereinafter called a rod extensometer of the type described).
Extensometers of the aforementioned design are mainly used for measuring movement in rock layers, e.g. in the building of tunnels and earth dams, but are also employed for measuring subsoil displacements in structures above the surface. The measuring operation normally consists of the measurement of the difference between various layers as regards their movement, so that in general a number of separate extensometers are sunk to different depths and their respective changes in position measured.
In practice there are two main types of measuring apparatus, that is the wire extensometer and the rod extensometer. In the former type a wire is taken from the point at which the extensometer is secured in the rock to the head of the extensometer, where it is deflected by a roller and loaded with a weight. Any movement occuring in the subsoil causes the wire to perform a corresponding movement on the roller, which is equipped with a length measuring attachment. Changes are thus recorded by a distance measuring operation. Wire extensometers are thus difficult to install, particularly when the securing points are very deep down, but difficulties are also encountered when a number of extensometric measurements have to be carried out in a limited space.
The rod-type extensometers described above have proved satisfactory in their place, the change in the position of the rod being measured by its pick-up element, such as a meter or a probe, on the extensometer head. However, the measuring operation, if carried out at great depths, again involved considerable expense and labour, as the rod system consisted of separate sections which had to be manipulated by hand, which were interconnected by couplings and which were built into the borehole one after the other. Under these circumstances it was not only the constructional outlay as such that proved a drawback but in many instances the time taken to insert the extensometer as well. There are many spheres of application, such as in tunnel building, after the termination of the base, when at a very early stage, for example during the first few hours and days, measurements are required which in their turn are decisive for the lining and safety operations in such structures.